I'd get a dropper, a shot glass, and do a dropper full of glycerin, and a dropper full of rum, whiskey, or other strong drink. 
Its really not advanced chemistry.  After its mixed take a dropper and examine how thick it is.  Too thick, add more liquid, to thin, add more glycerin.   When you think you have it right, bottle it. 

 Every 
e-liquid  manufacture has a different idea of how think the juice should be.  
I've also shaken bottles and examined how fast the micro bubbles rise to the surface.  After a minute the bubbles should be almost nonexistent, only the finest micro bubbles remain.  If you can count the bubbles, its too thick 8-o